Shallow trench isolation is widely used in integrated circuit technology as a means for electrically isolating different parts of the circuit from each other. As the name suggests, a relatively shallow trench is etched into the silicon surface and is then filled with a suitable dielectric material, most commonly silicon oxide.
In the interests of achieving maximum circuit density, the walls of these trenches are made relatively steep, sloping in from the vertical by no more than about 80 degrees. It has, however, been observed that if the walls of the filled trench intersect the surface at near 90.degree., a great deal of stress builds up at the silicon to oxide interface, leading to undesirable consequences such as cracking and poor gate oxide integrity.
Because of this stress problem, there has been a great deal of interest in providing trenches with a profile such that the walls are close to vertical at the trench bottom but, near the top, gradually flair out and approach the surface less steeply.
Takahashi et al. (U.S. Pat. No. 4,916,086 April 1990) accomplish top rounding by lining their trench walls with a layer of polysilicon that is oxidized just before the trench is filled in. Price et al. (U.S. Pat. No. 4,639,288 January 1987) produce a trench with a lower portion having steep walls and an upper portion having sloping walls by first performing an isotropic silicon etch and then following that with an anisotropic silicon etch.
Horioka et al. (U.S. Pat. No. 5,258,332 November 1993) achieve rounding of trench corners by using a mixture of fluorine gas combined with at least an equal amount of oxygen. Cox et al. (U.S. Pat. No. 4,417,448 January 1988) are concerned with producing deep trenches in silicon and have developed an etch chemistry based on oxygen, hydrogen chloride, and boron trichloride.